A tanker sank in the Sundarbans on December 9 spilling hundreds of thousands of litres of oil into the rivers and canals spread through the forest, triggering fears for dolphins and other endangered animals.
United Nations experts said last week they found "limited immediate impact" of the spill on the ecosystem, but recommended a permanent ban on the movement of transport boats and tankers through the Sundarbans.
However the government, under pressure from boat owners, said Wednesday that the main Shela river was being reopened until an alternate waterway was dredged, although oil tankers would remain banned.
Part of the Shela river was declared a sanctuary for Irrawaddy and Ganges river dolphins in 2011, one of the animals' few protected habitats.
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Last month's mishap prompted a massive outcry from environmentalists who demanded a permanent ban on using the river to transport goods between the country's west and southwest.
They said such spills could have a "catastrophic" impact on the 10,000 square kilometres forest's delicate mangrove ecosystem and its scores of rare animals.